Assessment of rapid low-cost isotope (δ15 N, δ18 O) analyses of nitrate in fruit extracts by Ti(III) reduction to differentiate organic from conventional production

Abstract

Rationale: The isotopic composition (δ¹⁵ N, δ¹⁸ O) of nitrate in fruits and vegetables differentiates organic from conventional food production practices. Organic systems do not use synthetic nitrate fertilizers high in ¹⁸ O and low in ¹⁵ N and thereby help reveal producers' fertilization claims. Isotope analyses of nitrate extracted from fruits and vegetables are done by bacterial reduction which is costly and by specialized laboratories. Rapid, low-cost methods are needed to promulgate nitrate isotope analyses of food products to support organic food product certification and to verify the authenticity of production claims.

Methods: Fresh strawberry samples were obtained from certified organic and conventional growers in Andalucía, Spain. We applied a new, rapid, one-step Ti(III) reduction method to convert the nitrate from strawberry extracts to N₂ O gas for headspace isotope analyses using isotope-ratio mass spectrometry. Using the Ti(III) reduction method, 70 samples, controls and references were prepared and analyzed for NO₃ ^- , δ¹⁵ N and δ¹⁸ O per 48 h. We also analyzed extracts and solids for anions and cations and for bulk δ¹⁵ N for multivariate chemometric evaluation.

Results: The Ti(III)-based isotope analyses of nitrate in strawberry extracts revealed clear differentiation between organic and conventional production with mean δ¹⁸ O and δ¹⁵ N values of +18.3 ± 1.2 ‰ and +17.6 ± 1.2 ‰ versus +28.2 ± 4.5 ‰ and +14.9 ± 3.0 ‰, respectively. The δ¹⁵ N of strawberry dry mass differed slightly (+3.0 ± 1.4 ‰ versus +4.0 ± 1.4 ‰) between organic and conventional samples, respectively. Chemometric analyses of nitrate isotopes and extract chemistry revealed that the δ¹⁸ O of nitrate along with δ¹⁵ N and Ca²⁺ fully differentiated organic from conventional strawberry production.

Conclusions: Our results show the Ti(III) reduction method provides a new low-cost and rapid analytical method to facilitate compound-specific δ¹⁵ N and δ¹⁸ O isotope analyses of nitrate in selected fruit types, and likely other food products, for the purposes of assessing nitrate fertilization practices of organic versus conventional production claims and to support authenticity investigations.